This application is related to Ser. No. 673,979, now abandoned, which is being filed concurrently herewith. The hydrogenation of unsaturated polymers is well known in the prior art. Usually a solution of the polymer in an inert solvent is contacted at elevated temperature with hydrogen under pressure in the presence of a heavy metal catalyst which usually comprises at least one transition metal catalyst comprising nickel, cobalt or iron, with or without aluminum or lithium. Specific techniques may be found in British Pat. No. 1,020,720.
Unsaturated polymers are hydrogenated for a variety of reasons. The presence of olefinic double bonds in the polymers makes them susceptible to oxygen attack and to deterioration by actinic radiation; saturation of olefinic double bonds greatly improves environmental stability. Hydrogenation may improve color. Polyethylene has been produced by hydrogenation of elastomeric polybutadiene (Ind. and Eng. Chem. 45, 1117-22 (1953), and Rubber Chem. and Tech. 35, 1052 (1962)). In certain block copolymers or homopolymers resistance to flow under stress when hot is improved by hydrogenating the aromatic rings to alicyclic rings. In other block copolymers made solely from dienes, good thermoplastic elastomers can be produced by hydrogenating all of the olefinic double bonds.
A common problem shared by all of these types of hydrogenated polymers is the deleterious effect of the small amounts of metal catalyst residues remaining after hydrogenation. The quantity of metal to be removed may be as high as 50,000 parts per million. The metal causes polymer deterioration by promoting reactions with air and actinic radiation, and must therefore be removed almost completely, e.g., to less than about 10 p.p.m. although higher limits may be adequate for some purposes. Filtration may be carried out first to remove much of the catalyst, but residual contamination is very difficult to remove by purely physical separation; chemical reaction and then separation are required.
Affter the hydrogenation reaction to saturate the double bonds in the polymer, metal catalyst residues remain and vigorous reaction, sometimes at elevated temperatures and for extended time, is required to dissolve them. Strong acids such as hydrochloric acid and sulfuric acid have been used in the past, but such mixtures are very corrosive and may have a deleterious effect on equipment used and on the hydrogenated polymer from which the metals are being removed.
U.S. Pat. No. 3,780,138 discloses a process which incorporates extraction with dilute citric acid. The method requires large volumes of extractant, relatively long extraction times and phase separation is not sharp. Currently practiced catalyst removal systems use an aqueous acid extraction system with 0.3-1% H.sub.2 SO.sub.4. The acid stream volumes are of comparable size to the polymer cement streams.
In a typical polymer synthesis, lithium salts of living polymer anion chains are terminated (quenched) with alcohol in the polymerization section at the conclusion of the polymerization reaction. This polymer cement, containing lithium alkoxides, is then moved downstream to the hydrogenation section where it is contacted with H.sub.2 and a Ni catalyst. The Ni catalyst is manufactured from Ni carboxylate salts and triethylaluminum. At the completion of the hydrogenation reaction, the polymer cement, containing catalyst, is sent to the catalyst extraction section where the Ni and Al are removed by contacting with, for example, dilute H.sub.3 PO.sub.4 or dilute H.sub.2 SO.sub.4. After phase separating the organic and aqueous streams, the polymer cement is forwarded to a finishing section. The aqueous stream can be sent to the effluent plant for metals recovery, for example, by precipitation, and for bio-treating to decompose entrained and dissolved organics. The recovered metal-containing-sludge is usually disposed of, for example, by trucking to a landfill.
It has now been discovered that the direct precipitation of metals from the organic polymer cement phase is possible using an oxidant and dicarboxylic acid. The approach described here combines extraction and precipitation steps using di-carboxylic acids to both extract and precipitate metals directly from polymer cements. After precipitation of the catalyst, the cement could be filtered and sent directly to finishing. There would be no aqueous phase and thus no bio-treating load from the catalyst removal unit. Likewise, the metals removal operation would be done in the catalyst removal section, not in the effluent treatment area.
The method of the present invention would have several advantages over the method used commercially at present. The acid volume used could be signicantly smaller than when using the conventional dilute H.sub.2 SO.sub.4 system or the concentrated organic acid method. With a rapid direct extraction, most of the extraction unit facilities could be eliminated, e.g., extraction vessels and phase separators. With modest mixing requirements, it may be possible to substitute static mixers for mechanical mixers in existing facilities. Cheaper metals, e.g., 316 or 304 stainless steel could be used for the extraction vessels and phase separators in the downsized and simplified facility and would represent a substantial capital savings. However, the greatest advantage is that the method is accomplished in a single phase, thus eliminating all of the 2 phase procedures, e.g., settling and separating.
In selecting a suitable agent to remove metallic residues from the hydrogenated polymers, a complex and interrelated set of criteria should be considered as desirable: The agent chosen should be substantially inert toward the polymer and polymer solvent; it should be capable of forming a compound which is insoluble in the organic solvent. It preferably reacts with iron (often present as a contaminant from equipment or water); capable of forming an in-soluble compound with nickel or cobalt and aluminum (present as hydrogenation catalyst residues over a wide pH range); preferably when lithium initiated polymers are concerned, it should be capable of reacting with any lithium residues which may remain after forming the original polymers. Agents which fail to satisfy any one of the above criteria may be regarded as unsatisfactory for the present purpose.